A common organic EL device comprises an anode composed of ITO or the like provided on a substrate, an organic layer provided on the anode, and a cathode provided thereon. The organic layer has a constitution in which a hole-injecting layer, a hole-transporting layer and an emitting layer are stacked in this sequence from the anode, for example. In the organic EL device having such a constitution, light generated when electrons injected from the cathode and holes injected from the anode are recombined in the emitting layer is outcoupled from the substrate side.
The life of an organic EL device is generally determined by the amount of injected charges. This leads to a problem that the life is shortened when a driving current is increased in order to increase an initial luminance.
In order to solve the problem, it is necessary to increase the luminance without changing the driving current, i.e. to improve the efficiency, or to realize a device constitution which does not deteriorate the luminance even when the driving current is reduced.
For the above-mentioned device constitution, a constitution in which plural emitting units each comprising organic layers including at least an emitting layer are stacked between an anode and a cathode via insulating charge-generating layers (MPE device) has been proposed. Here, the charge-generating layer is a layer which, when a voltage is applied, serves to inject holes to an emitting unit arranged on the side of a charge-generating layer near to a cathode, and on the other hand, serves to inject electrons to an emitting unit arranged on the side of the charge-generating layer near to an anode.
In a stacked-type organic EL device in which emitting units are stacked via charge-generating layers, it is known that the luminance [cd/A] can be ideally increased two-fold with the luminous efficiency [Im/W] being unchanged in the case where two emitting units are stacked, and the luminance [cd/A] can be ideally increased three-fold with the luminous efficiency [Im/W] being unchanged in the case where three emitting units are stacked.
For the MPE device, Patent Document 1 and 3 disclose a device in which a transparent conductor (ITO, IZO or the like) is used in a charge-generating layer, for example.
Patent Document 2 discloses a device in which vanadium oxide (V2O5) or rhenium (VII) oxide (Re2O7) is used in a charge-generating layer. Patent Document 4 discloses a device in which a metal oxide such as molybdenum oxide (MoO3) or a metal salt such as ferric chloride (FeCl3) is used in a charge-generating layer. Patent Document 5 discloses a device in which a combination of an N-doped layer and a P-doped layer is used in a charge-generating layer. Patent Document 6 discloses a device in which a phthalocyanine compound is used in a charge-generating layer. Patent Document 7 discloses a device in which an electron-accepting organic substance such as hexaazatriphenylene (HAT) and F4TCNQ described in Patent Document 2 is used in a charge-generating layer.
As can be seen from the above, although various materials are used in a charge-generating layer, a conventional charge-generating layer still has some problems to be solved.
Specifically, an inorganic substance such as a metal oxide requires high deposition temperatures at film-formation, thereby decreasing the efficiency of film-formation process and thus lowering mass productivity.
Moreover, since a transparent conductor such as ITO has a high electrical conductivity, current leakage may occur between pixels through a charge-generating layer. Therefore, when desired pixels are allowed to emit, adjacent pixels may emit. The phenomenon becomes a problem in a display in which an organic EL device is allowed to emit white light, and each color of RGB is outcoupled through each color filter provided on the device. That is, the color purity is remarkably lowered by color mixing due to the emission of adjacent pixels, thereby deteriorating color reproducibility.
In addition, there is a fear that when a transparent conductor is formed into a film by sputtering or the like, generated plasma particles damage an organic layer serving as a base.
On the other hand, when an organic compound is used, since the temperature of film formation is normally low and thus plasma particles are not generated, the efficiency of film formation process and the mass productivity is excellent. When an electron-accepting organic compound is used, in particular, HAT is used, as shown in Patent Document 7, a device superior in terms of properties such as efficiency, voltage, lifetime or the like can be obtained. However, HAT itself has a high conductivity, current leakage may occur between pixels through a charge-generating layer as in the case of the above-mentioned transparent conductor.